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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.line_based"></a><a class="link" href="line_based.html" title="Line-Based Operations">Line-Based Operations</a>
</h4></div></div></div>
<p>
          Many commonly-used internet protocols are line-based, which means that
          they have protocol elements that are delimited by the character sequence
          <code class="computeroutput"><span class="string">"\r\n"</span></code>. Examples
          include HTTP, SMTP and FTP. To more easily permit the implementation of
          line-based protocols, as well as other protocols that use delimiters, Boost.Asio
          includes the functions <code class="computeroutput"><span class="identifier">read_until</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">async_read_until</span><span class="special">()</span></code>.
        </p>
<p>
          The following example illustrates the use of <code class="computeroutput"><span class="identifier">async_read_until</span><span class="special">()</span></code> in an HTTP server, to receive the first
          line of an HTTP request from a client:
        </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">http_connection</span>
<span class="special">{</span>
  <span class="special">...</span>

  <span class="keyword">void</span> <span class="identifier">start</span><span class="special">()</span>
  <span class="special">{</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read_until</span><span class="special">(</span><span class="identifier">socket_</span><span class="special">,</span> <span class="identifier">data_</span><span class="special">,</span> <span class="string">"\r\n"</span><span class="special">,</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">http_connection</span><span class="special">::</span><span class="identifier">handle_request_line</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">_1</span><span class="special">));</span>
  <span class="special">}</span>

  <span class="keyword">void</span> <span class="identifier">handle_request_line</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">ec</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">method</span><span class="special">,</span> <span class="identifier">uri</span><span class="special">,</span> <span class="identifier">version</span><span class="special">;</span>
      <span class="keyword">char</span> <span class="identifier">sp1</span><span class="special">,</span> <span class="identifier">sp2</span><span class="special">,</span> <span class="identifier">cr</span><span class="special">,</span> <span class="identifier">lf</span><span class="special">;</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="identifier">is</span><span class="special">(&amp;</span><span class="identifier">data_</span><span class="special">);</span>
      <span class="identifier">is</span><span class="special">.</span><span class="identifier">unsetf</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">skipws</span><span class="special">);</span>
      <span class="identifier">is</span> <span class="special">&gt;&gt;</span> <span class="identifier">method</span> <span class="special">&gt;&gt;</span> <span class="identifier">sp1</span> <span class="special">&gt;&gt;</span> <span class="identifier">uri</span> <span class="special">&gt;&gt;</span> <span class="identifier">sp2</span> <span class="special">&gt;&gt;</span> <span class="identifier">version</span> <span class="special">&gt;&gt;</span> <span class="identifier">cr</span> <span class="special">&gt;&gt;</span> <span class="identifier">lf</span><span class="special">;</span>
      <span class="special">...</span>
    <span class="special">}</span>
  <span class="special">}</span>

  <span class="special">...</span>

  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">socket_</span><span class="special">;</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">streambuf</span> <span class="identifier">data_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
          The <code class="computeroutput"><span class="identifier">streambuf</span></code> data member
          serves as a place to store the data that has been read from the socket
          before it is searched for the delimiter. It is important to remember that
          there may be additional data <span class="emphasis"><em>after</em></span> the delimiter.
          This surplus data should be left in the <code class="computeroutput"><span class="identifier">streambuf</span></code>
          so that it may be inspected by a subsequent call to <code class="computeroutput"><span class="identifier">read_until</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">async_read_until</span><span class="special">()</span></code>.
        </p>
<p>
          The delimiters may be specified as a single <code class="computeroutput"><span class="keyword">char</span></code>,
          a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> or a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span></code>.
          The <code class="computeroutput"><span class="identifier">read_until</span><span class="special">()</span></code>
          and <code class="computeroutput"><span class="identifier">async_read_until</span><span class="special">()</span></code>
          functions also include overloads that accept a user-defined function object
          called a match condition. For example, to read data into a streambuf until
          whitespace is encountered:
        </p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffers_iterator</span><span class="special">&lt;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">streambuf</span><span class="special">::</span><span class="identifier">const_buffers_type</span><span class="special">&gt;</span> <span class="identifier">iterator</span><span class="special">;</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">iterator</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&gt;</span>
<span class="identifier">match_whitespace</span><span class="special">(</span><span class="identifier">iterator</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">iterator</span> <span class="identifier">end</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">begin</span><span class="special">;</span>
  <span class="keyword">while</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">)</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">isspace</span><span class="special">(*</span><span class="identifier">i</span><span class="special">++))</span>
      <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="keyword">true</span><span class="special">);</span>
  <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="keyword">false</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">...</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">streambuf</span> <span class="identifier">b</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">read_until</span><span class="special">(</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">match_whitespace</span><span class="special">);</span>
</pre>
<p>
          To read data into a streambuf until a matching character is found:
        </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">match_char</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
  <span class="keyword">explicit</span> <span class="identifier">match_char</span><span class="special">(</span><span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">c_</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">{}</span>

  <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&gt;</span> <span class="keyword">operator</span><span class="special">()(</span>
      <span class="identifier">Iterator</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">Iterator</span> <span class="identifier">end</span><span class="special">)</span> <span class="keyword">const</span>
  <span class="special">{</span>
    <span class="identifier">Iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">begin</span><span class="special">;</span>
    <span class="keyword">while</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">)</span>
      <span class="keyword">if</span> <span class="special">(</span><span class="identifier">c_</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">i</span><span class="special">++)</span>
        <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="keyword">true</span><span class="special">);</span>
    <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="keyword">false</span><span class="special">);</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="keyword">char</span> <span class="identifier">c_</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">asio</span> <span class="special">{</span>
  <span class="keyword">template</span> <span class="special">&lt;&gt;</span> <span class="keyword">struct</span> <span class="identifier">is_match_condition</span><span class="special">&lt;</span><span class="identifier">match_char</span><span class="special">&gt;</span>
    <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">true_type</span> <span class="special">{};</span>
<span class="special">}</span> <span class="special">}</span> <span class="comment">// namespace boost::asio</span>
<span class="special">...</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">streambuf</span> <span class="identifier">b</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">read_until</span><span class="special">(</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">match_char</span><span class="special">(</span><span class="char">'a'</span><span class="special">));</span>
</pre>
<p>
          The <code class="computeroutput"><span class="identifier">is_match_condition</span><span class="special">&lt;&gt;</span></code> type trait automatically evaluates
          to true for functions, and for function objects with a nested <code class="computeroutput"><span class="identifier">result_type</span></code> typedef. For other types
          the trait must be explicitly specialised, as shown above.
        </p>
<h6>
<a name="boost_asio.overview.core.line_based.h0"></a>
          <span class="phrase"><a name="boost_asio.overview.core.line_based.see_also"></a></span><a class="link" href="line_based.html#boost_asio.overview.core.line_based.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/async_read_until.html" title="async_read_until">async_read_until()</a>,
          <a class="link" href="../../reference/is_match_condition.html" title="is_match_condition">is_match_condition</a>,
          <a class="link" href="../../reference/read_until.html" title="read_until">read_until()</a>, <a class="link" href="../../reference/streambuf.html" title="streambuf">streambuf</a>, <a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.http_client">HTTP
          client example</a>.
        </p>
</div>
<div class="copyright-footer">Copyright © 2003-2024 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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